This invention lies in the field of cardiac device systems and methods and, in particular, implantable systems that have the capacity to acquire data from the patient""s heart and to process such data to provide information concerning the patient""s heart. More specifically, this invention is directed toward providing an indication of degree of patient heart failure based on information derived from heart signals.
The utilization of pacemakers for diagnostic functions as well as pacing and cardioversion therapy has increased along with the capacity of pacemaker devices to collect and store data. Several generations of pacing systems have incorporated schemes for collecting and storing data derived from the patient""s heart, as well as data representative of the history of pacemaker functions. This data can be organized to provide an historical picture of the patient""s heart, coded, and downloaded to an external device for analysis by a physician. For example, the physician can be provided with cardiac rate histories, instances of ectopic beats, tachycardia episodes, and the like.
More recently, increased attention has been paid to the area of heart failure (alternatively referred to simply as xe2x80x9cHFxe2x80x9d) and mechanisms for detecting and treating this condition. As with many other diseases, early detection can provide increased opportunities for inhibiting onset and/or for effective treatment. The advent of bi-ventricular pacing systems has provided increased opportunities for treating at least some forms of heart failure. However, initiation of any form of treatment depends upon the availability of accurate information concerning the condition of the patient""s heart.
One recognized indicator of heart failure is prolongation of the QT interval (or, simply, xe2x80x9cQTxe2x80x9d), particularly at low rates associated with rest. It is known that QT interval increases with lower rates and decreases with higher rates. This is because QT has a component that increases directly with the cardiac interval, as well as a stress dependent variation due to the autonomic nervous system. Prolongation, as the term is used here, refers not to the normal increase in QT at lower rates, but to an increase beyond the normal that is found to occur in cases of established heart failure. The literature recognizes that patients with heart failure are characterized with prolongation of the QT interval at low heart rates, although not at high heart rates. xe2x80x9cProlongation of the QT interval in heart failure occurs at low but not at high heart rates.xe2x80x9d Davey, Barlow and Hart, Clin. Sci (Colch) May 2000; 98(5): 603-10. The disclosed investigations found that QTc (corrected QT) intervals at rest were significantly longer in heart failure patients, and were associated principally with impairment of left ventricular systolic function.
The use of variations in QT interval or of corrected QTc as an indicator of heart failure has limitations, as stated in the above referenced publication. Although prolongation of QT interval at rest seems to be an excellent indicator of established heart failure, changes in QT by itself have not been shown to provide a reliable prognosis for the patient who is proceeding toward heart failure. What is needed is more information, collected over time, to suggest changes that can be used by the physician, or compared automatically to benchmarks, to more reliably point to onset of heart failure.
U.S. Pat. No. 5,792,197, Nappholz, discloses an implantable rate responsive pacemaker that uses a physiological demand parameter to determine a classification of the degree of patient heart failure. In the illustrated embodiment, minute volume is monitored and used as a physiological demand parameter. Variations of the parameter corresponding to different levels of activity are obtained, and differences in the parameter over time are used to determine the degree of heart failure. QT interval and cardiac contractibility are mentioned as possible parameters. However, the patent reference does not indicate specifically how to utilize QT information, nor does it suggest the range of possibilities for obtaining predictive information from QT variations. Other patents, as listed in Table 1 below, deal with the subject of identifying cardiac events and trying to determine the patient""s cardiac condition, but do not optimize the information available from sensed heart signals. Accordingly, there has remained a need to more fully utilize information inherent in QT variations and to utilize that information for prognosis of heart failure.
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the resent invention.
This invention addresses the detection and progression of heart failure (HF), and particularly the detection of HF at an early enough stage to enable treatment of the patient with drugs or, in some cases, pacing therapies. The ability to accurately predict or recognize HF at an early stage may, in many cases, enable management of the condition in order to slow down its progression, and prolong a good quality of life.
It is an object of this invention to provide an implantable medical device, and a method of treatment using such device, that optimally utilizes the information inherent in patient cardiac signals to monitor onset of and progression toward heart failure, and to provide predictive indications of heart failure. It is a specific object to utilize dynamic variations of QT interval data, obtained from sensed and also paced cardiac signals. The data is utilized to predict heart failure and to provide indications of current patient degree of heart failure. The dynamic variations are referred to as dynamic QT parameters, and represent variations of QT with rate, i.e., QT(RR), that are measured as patient heart rate is first raised and then lowered. Such dynamic variations provide information indicative of HF that is not available solely from measurement of long term changes of QT at rest.
It is another object of this invention to provide an implantable medical device system and method of treatment wherein QT data is obtained from both right and left ventricles, and stored over time. The QT data from both ventricles provides an important comparison for analysis of patient cardiac condition, particularly the onset of HF.
In accord with the above objective, there is provided a system and method that incorporates an implantable medical device, and that obtains comprehensive data from patient cardiac signals, and in particular the QRS-T portions of the signals. The system analyzes the data and provides current and prognostic indications the patient""s cardiac condition. The system and treatment method of this invention utilize not only QT interval data, but also make available data relating to different portions, or segments of the QT complex, i.e., the QRS width, the ST segment, and the T wave width. Each of these segments varies with time and exercise, and in some patients may contain useful prognostic data that is not obtainable just from a measurement of QT interval. As used herein, QT data refers to the QT interval, or QTc, as well as the segment data. In addition, the system and method of this invention are designed to obtain and use dynamic QT data over a range of patient heart rates, so as to capture additional information that is not available simply from the measurement of QT interval changes at rest.
In one embodiment of the invention, the dynamic rate range of QT interval is obtained by determining variation of patient heart rate with respect to one or more predetermined QT intervals as the patient exercises to raise rate and then rests, permitting rate to recover toward a rest rate. The difference between the rate found at a given QT interval as the patient heart rate rises in response to exercise, and the rate found as the patient recovers toward rest, represents a rate range that will vary in patients who are progressing toward a higher degree of heart failure. The system obtains and stores such dynamic rate range data, and provides it for indication of degree of patient heart failure.
In another embodiment of the invention, one which is a complement of rate range, dynamic QT hysteresis is measured and utilized for determination of the patient""s cardiac status. In this embodiment, the patient is taken through a cycle of exercise and recovery, and QT interval is measured at one or more predetermined heart rates as rate rises in response to exercise, and again at the same rate (or rates) after the patient stops exercise and recovers. In this situation, QT interval is found to be higher as rate climbs in response to exercise than it is at the same rate as the patient recovers. This difference, or QT hysteresis, varies with time for a patient, and thus is a dynamic variable that, like the dynamic rate range, provides useful data concerning changes in the patient""s cardiac condition. Dynamic QT hysteresis measurements may utilize the entire QT interval, or any combination of the segments of the QRS-T signal.
In yet another embodiment of the invention, dynamic QT directrix data is obtained that represents differences in the slope of the QT (RR) curve around a given rate during exercise and recovery. Thus, as the patient heart rate increases due to exercise, a measure of QT slope around one or more trigger rates is determined, and after cessation of exercise and recovery of rate toward rest, the slope is again determined around the same rate or rates. This slope information, like the rate range and QT hysteresis values, is dynamic with time for a patient who is advancing to greater degrees of heart failure, and thus provides further useful data to inform as to the patient""s progression.
By incorporation of one or more of the above features, the system and method of this invention provide an enhanced capability for acquiring and analyzing data from which a patient""s progression toward heart failure can be tracked. The system of this invention is preferably embodied with a microprocessor-based implantable device, and utilizes DSP technology to provide signal data in useful form for analysis. The system advantageously discriminates against ectopic beats that would interfere with provision of desired data, and also provides for accurate separation of the QRS-T segment data, for indicator use. In another embodiment, an analysis algorithm may use just the QRS-T data for providing indications of degree of heart failure, or may utilize such QRS-T data in combination with the dynamic data.